The present invention relates to a method for making a sensing element and a sensing element used in a catalytic combustible gas sensor and more particularly to a poison resistant sensing element of the resistive type used in atmospheres containing poisons, such as silicones, halogenated hydrocarbons, and organometallics, and a method for making the same.
The present invention is applicable generally to combustible gas sensors of the imbedded coiled wire filament type preferably of the kind described in U.S. Pat. Nos. 3,959,764 and 4,068,021, that is, a helically coiled filament coated with a refractory material, generally applied in a liquid form, which is then heated, e.g. in a furnace, so that the coating matures during the heating and sintering takes place to produce a dense sheath. The sintering process shrinks the coating and, at the same time, compresses the helical coil, to form a dense coated helical coil filament with an internal passage therethrough. Over this dense coating, a platinum catalyst is applied to oxidize combustible gases. A difficulty which has been experienced with sensors as described above is that they rapidly lose their response level when exposed to a catalytic poison.
Further, the problem with prior art devices, such as bead-type sensing elements as described in U.S. Pat. Nos. 3,200,011 and 4,123,225 is that they require immediate recalibration or replacement after exposure to a catalytic poison. These sensors operate very successfully in clean atmospheres, i.e. without loss of sensitivity, however, use in more hostile industrial environments results in irreversible loss of sensitivity or poisoning.